1. Field of the Invention
The present invention is directed to rocket propulsion, and, more particularly, to peristaltic electric injectors having a multiplicity of waveplates electrically undulated by shear transducer action.
2. Description of Related Art
The preponderance of known rocket injectors use separate fuel and oxidizer pumps to provide a feed pressure greater than the combination of injector pressure drop and of the combustion pressure in the rocket chamber, thereby maintaining propellant flow through generally fixed injection nozzles. An alternate design of rocket engines obviates pumps by supplying propellants from tanks that are strong enough to sustain pressures greater than those prevailing at the inlets of the injector orifices. Typically, there is a separate component corresponding to each function required by the injection system of a rocket motor. Whether pump-fed or pressure-fed, rocket motor systems are often heavier and more complex than those systems that combine several functions into each component.
The thrust from a given rocket engine, given adequate nozzle strength, depends largely on the combustion pressure. The injector pressure must be greater than the combustion chamber pressure in order to prevent retrograde flow of reacting propellants into the injector orifices. In general, increasing injector pressure increases the thrust of the rocket engine, while increasing the engine's efficiency. As is well understood, the size and weight of the injector pressurizing means increases as a power function of the injection pressure. At higher pressures, pump weight and bulk, or alternatively, tank weight and bulk, increase to values that are detrimental to the performance of the whole propulsion system, disregarding for the sake of discussion the weight, bulk, and complexity of the nozzle itself.
Current rocket engine systems, using improved materials and manufacturing techniques, have increased running time, combustion chamber pressure and temperature, and increase the quantity of propellant that can be reacted during a single prolonged thrust cycle. It is therefore a trend that propellant tank size, relative to the size of the rocket engine itself, is increasing. As propellant tank size increases it becomes advantageous to adopt the pump-fed propellant supply system rather than the pressure-fed system employing strong and heavy propellant tanks.
Current rocket engine systems also have more complex start-up, run, and shut-down valve sequences than their predecessors, entailing the use of a relatively large number of control valves, pressure, temperature and flow sensors and other related components. In addition, present rocket propulsion systems for many applications operate more effectively when thrust is controllably variable over a wide range. Considerable design and development effort has resulted in rocket engine systems that provide good thrust range, for example, 20 to 1, accompanied by relatively little loss of operating efficiency at thrust values that differ substantially from a predetermined optimum value somewhere within the range of thrust.
Adjusting thrust entails, in part, the adjustment of the flow rate of propellants passed by the injectors. Among the many possible schemes for flow rate adjustment, a common one maintains propellant pressure at a prescribed value while propellant flow rates are controlled by valves having variable mass flow admittance. The task of coordinating at least one variable orifice valve for each propellant stream is made more difficult by the inherent nonlinearity of mass flow through the valve as a function of valve actuator position. Mass flow is also dependent on the current state of propellants, changes of state of propellants with time and flow, and on many other factors that leave unmitigated a complex set of problems arising from the operation of high efficiency, variable-thrust rocket engines.
Of the known pumps used for rocket engine injector pressurization, a common one is the turbopump, a turbine empowered by a combusted portion of the propellants, connected by a shared shaft to a turbine pump that provides the desired propellant pressure and flow. At least one turbopump is used for each propellant stream. In a diverse class of rocket engines, two pumps are used for each propellant stream, one for low pressure and one for high. Maintaining a desired behavior of four pumps further complicates the control apparatus. In the absence of proper propellant delivery, the chemical reaction will proceed with less than optimum performance. Further, improper balance may shorten or even terminate the useful life of the system.
Turbopumps are usually of the high speed rotating variety wherein the inertia of the combined turbine wheels and the shared shaft preclude timely adjustment of propellant flow rate and pressure by varying turbopump speed. Generally, the turbopumps are capable of pressure and mass flow delivery exceeding the maximum anticipated needs of the injector system, the excess being attenuated by the aforementioned valves. The energy consumed to maintain the operational margin, if dissipated, further reduces the system efficiency compared to an ideal system in which the pressurizing means supplies exactly the pressure and flow rate required at every instant of operation.
Present rocket engines have a relatively large number of injector orifices that further improve engine performance by more effectively atomizing and mixing propellants, and by assuring more uniform combustion through disbursed propellant impingement. The energy density in a rocket's combustion chamber is perhaps, short of bombs and other explosive devices, the highest of all known machines. A maximally efficient engine operates in the very threshold of self destruction. Therefore, uniformity of combustion prolongs the life of a given rocket engine because combustion temperatures are more uniform, and variations of peak pressure, measured locally, are restricted to a narrower range. Greater combustion uniformity results in significantly more quiet engine operation, wherein quiet refers to the suppression of vector sums of acoustic maxima that are known to disrupt rocket engines.
A diverse class of rocket engines comprises systems expressly designed for operation in dense fluids, such as engines for submarines, torpedoes, and other marine vessels. Generally, the thrust generated by a given engine is reduced in proportion to the ambient pressure against which the nozzle exhaust must act. Under water, particularly at great depths, much higher nozzle pressures are required to achieve desirably high values of thrust. High nozzle pressure requires higher combustion chamber pressure, which, in turn, requires higher injection pressure. It is therefore advantageous that the injector pressurizing means for immersed rocket engines be more capable and more easily controlled than their space-borne counterparts.
The injectors of present engines, besides being numerous, have flow orifices that are small enough to be occluded by relatively small particles. Despite complex preventive measures applied to the handling and delivery of propellants, and in the cleaning and storage of engine systems, at least some type of filter or sieve in each propellant supply line is an advantage. In combination with a filter, a means of crushing particles entrained in the propellant stream would allow passage of the fragments through injector orifices without occlusion (chemically non-reactive particles assumed). However, filters and particle crushers reduce rocket system efficiency by reducing mass flow and delivery pressure, while dissipating;a significant fraction of the propellant-borne kinetic and potential energy as frictional heat.
An advantageous injector pressurization apparatus combines into a single component much of the apparatus for pressurization, filtering, particle fragmentation, mass flow adjustment, and pressure control needed for efficient rocket operation over a desirably wide range of ambient conditions and range of thrust.
A diverse class of piezoelectric pumps uses a piston or other displacement means, actuated by piezoelectric action, to move fluid wherein the displacement means generally oscillates while at least two fluid valves prevent most of the displaced fluid from moving in a direction other than the desired one. Typical of this class of pumps is a piezoelectric automobile fuel injector by Takahashi, U.S. Pat. No. 4,803,393 in which piezoelectric thickness mode action is transmitted hydraulically to the displacing means by way of a diaphragm or a bellows. The life of a pump of this type is shortened by rubbing at contacts between seals and sliding surfaces, between the displacer and cylinder, and by fatigue of valves and, if used, of flexible membrane seals. Such devices are designed for pulsatile fluid delivery and peak pressures comparable to the pressures needed for rocket injectors. However, the pulse repetition rate is limited to the operating speed of the check valves and the recovery time of the piezoelectric displacer actuator. Lacking a sufficiently rapid pulse rate, this class of pumps provides relatively low average delivery pressure, in most cases too low to pressurize the injectors of present rocket engines. In addition, pulsatile propellant delivery has pump cycle portions during which retrograde propellant flow may occur, constituting a dangerous condition in which combustion internal to the injectors and their manifolds is likely.
The mass flow provided by the largest known pulsatile piezoelectric pumps is insufficient to pressurize all but the tiniest rocket engines. A moderate to large engine may use a large number of these pumps. A rocket engine using the design architecture having such a large number of independently operated pumps, each pump supplying an injection orifice (or pair of same), has advantages such as: the injector system would be relatively tolerant of isolated pump failures, and thrust value is easily varied by activating a portion of the orifices. Unfortunately, the random failure of, for example, an oxidizer injector pump, must be sensed in order to terminate the operation of the corresponding fuel injector pump in order to prevent, at least, the loss of desired combustion stoichiometry, and , at most, participation by engine parts in the combustion process.
Thickness mode piezoelectric stacks of pumps polarize ferroelectric material in the direction of the applied electric field. If an electric field is applied in the reverse direction, the polarization will be reduced, destroyed, or reversed in direction, all of which reduce the performance of the piezoelectric elements. Therefore, thickness stacks are usually operated with monopolar electric potentials. Electric drive means that provide monopolar electric signals are more complicated than bipolar electric drive means because of the need for floating power supplies, additional insulation and the like. Given equivalent geometry and similar electric field intensity, a thickness stack produces half the mechanical stroke that would otherwise be available if both electric drive potentials and piezoelectric deformations were bipolar. Furthermore, relatively high net electric potentials reside in monopolar apparatus during operation, a state more conducive to electrical breakdown and shock hazard.
Known piezoelectric pulsatile pumps store a large portion of the circulating energy in the form of elastic deformation of the pump body and in the mechanisms attaching the displacing means to the piezoelectric actuator stack. Additional energy is stored in the piezoelectric elements in the form of electric charge. These energies are generally only restored to the pump system between portions of the pumping cycles during which useful work is performed on the fluid. Energies that are not returned to the pump system are typically dissipated as mechanical heat of friction and electrical heat of resistance, causing operation at reduced electromechanical efficiency. Internal dissipation suffers the apparatus a shorter life because of the accompanying higher operating temperatures.
Another life shortening mechanism is fatigue and shock loading. For example, the pump drive means of Mitsuyasu, U.S. Pat. No. 4,688,536, charges piezoelectric elements of a stack in electrical parallel, and then discharges them in a sequence through inductive-capacitive circuits to provide a desired injection pressure waveform, and to ameliorate the mechanical shock that otherwise obtains when piezoelectric portions were discharged in unison. Pump action is designed to be pulsatile and somewhat abrupt as required by the application of injecting fuel for automotive engines.
A disadvantage of pulsatile pump operation, aside from those previously cited, is the relatively inefficient use of available electric power. An advantage of an ideal rocket injector pump is continuous flow, or if not continuous, a flow that comprises a sufficient number of parallel fluid contributions so as to be virtually indistinguishable from continuous. Flow that is continuous or quasicontinuous is efficiently produced by an ideal pump that uses a multiplicity of electrically resonant power sources. Resonance allows the preponderance of temporarily stored electrical energy to be recycled in the drive system rather than dissipated as resistive heat. Electrical power consumption by the ideal drive system is relegated to that power portion directly converted to mechanical work done on the fluid. Controlling the ideal resonant continuous-flow pump is tantamount to controlling the electrical parameters of electromechanical resonance.